Relay Imaging Research Articles

Systematic error of a large dynamic range aberrometer

Shack-Hartmann aberrometers are routinely used for measuring ocular aberrations. In one configuration, the intermediate images of the Shack-Hartmann spots formed by the lenslet array are relayed by an imaging lens onto a sensor. A systematic introduction of spherical aberration that is strongly related to the power error (defocus) of the incident wavefront is observed in this configuration. We found that the largest component of this error is induced by the pupil aberration of the imaging relay lens. Some simulations and experimental results are demonstrated.

Concept of neutral gain modules for power scaling of thin-disk lasers

We present a concept for power scaling of high brightness solid state lasers, which introduces so called gain modules containing the laser active media. These modules can be inserted as relay imaging optical systems in any type of laser resonators. The gain modules are optically neutral; hence, power scaling can be provided inserting several modules. Here, we provide the basic functional units of gain modules as well as an exemplary experimental implementation in thin-disk lasers with dynamically stable resonators. On the basis of these studies, more than 1 kW output power with an averaged M2=2.6 could be demonstrated using two disks. Experiments with four disks are in preparation.

Compact 300-J/300-GW Frequency-Doubled Neodymium Glass Laser—Part I: Limiting Power by Self-Focusing

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> Small-scale self-focusing is the main cause of power limitation in nanosecond Nd:glass lasers. We report pioneer observation in experiment of laser beam noise amplification for <formula formulatype="inline"><tex Notation="TeX">$B$</tex> </formula>-integral (nonlinear phase) order of unity, i.e., without destruction of nonlinear medium. Analytical dependences of noise amplification coefficient on the <formula formulatype="inline"><tex Notation="TeX">$B$</tex></formula>-integral have been obtained for one nonlinear medium as well as for two nonlinear media separated by an air gap with image relay. A simple analytical dependence has been found for an optimal distance between two nonlinear media for which the maximum allowable value of <formula formulatype="inline"><tex Notation="TeX">$B$</tex> </formula>-integral increases from 2.7 to 4.3. The maximum attainable energy in neodymium-phosphate glass lasers with rod amplifiers 10 cm in diameter is 400 J with a pulse duration of 1 ns. </para>

Construction of an integrated Raman- and angular-scattering microscope

We report on the construction of a multimodal microscope platform capable of gathering both elastically and inelastically scattered light from a 38 mum(2) region in both epi- and transillumination geometries. Simultaneous monitoring of elastic and inelastic scattering from a microscopic region allows noninvasive characterization of the chemistry and morphology of a living sample without the need for exogenous dyes or labels, thus allowing measurements to be made longitudinally in time on the same sample as it evolves naturally. A sample is illuminated either from above or below with a focused 785 nm TEM(00) mode laser beam, with elastic and inelastic scattering collected by two separate measurement arms. The measurements may be made either simultaneously, if identical illumination geometries are used, or sequentially, if the two modalities utilize opposing illumination paths. In the inelastic arm, Stokes-shifted light is dispersed by a spectrograph onto a charge-coupled device (CCD) array. In the elastic scattering collection arm, a relay system images the microscope's back aperture onto a CCD array. Postprocessing of the inelastic scattering to remove fluorescence signals yields high quality Raman spectra that report on the sample's chemical makeup. Comparison of the elastically scattered pupil images to generalized Lorenz-Mie theory yields estimated size distributions of scatterers within the sample.

270-W 15-kHz MOPA System Based on Side-pumped Rod-type Nd:YAG Gain Modules

We have developed a 270-W 15-kHz MOPA system based on side-pumped rod-type Nd:YAG gain modules. The master oscillator is a 3-W 15-kHz <TEX>$TEM_{00}$</TEX> <TEX>$Nd:YVO_4$</TEX> laser with a pulse duration of 30 ns. To preserve the high beam quality during the amplification, we use image relay and polarization rotation which can simultaneously compensate for thermal lensing and thermal birefringence generated in the rod-type gain modules. After the amplification to 270 W with six rod-type gain modules, the beam quality factor (<TEX>$M^2$</TEX>) of the amplified laser beam is 5-10, and the pulse duration is maintained at 30 ns.

Dynamic optical modulation of an electron beam on a photocathode RF gun: Toward intensity-modulated radiation therapy (IMRT)

Abstract In intensity-modulated radiation therapy (IMRT), the aim is to deliver reduced doses of radiation to normal tissue. As a step toward IMRT, we examined dynamic optical modulation of an electron beam produced by a photocathode RF gun. Images on photomasks were transferred onto a photocathode by relay imaging. The resulting beam was controlled by a remote mirror. The modulated electron beam maintained its shape on acceleration, had a fine spatial resolution, and could be moved dynamically by optical methods.

Validation of a rotational coherent anti-Stokes Raman spectroscopy model for carbon dioxide using high-resolution detection in the temperature range 294-1143 K

Experiments were performed in the temperature range of 294-1143 K in pure CO(2) using high-resolution rotational coherent anti-Stokes Raman spectroscopy (CARS), in the dual-broadband approach. Experimental single-shot spectra were recorded with high spectral resolution using a single-mode Nd:YAG laser and a relay imaging lens system on the exit of a 1 m spectrometer. A theoretical rotational CARS model for CO(2) was developed for evaluation of the experimental spectra. The evaluated mean temperatures of the recorded single-shot dual-broadband rotational coherent anti-Stokes Raman spectroscopy (DB-RCARS) spectra using this model showed good agreement with thermocouple temperatures, and the relative standard deviation of evaluated single-shot temperatures was generally 2-3%. Simultaneous thermometry and relative CO(2)/N(2)-concentration measurements were demonstrated in the product gas of premixed laminar CO/air flames at atmospheric pressure. Although the model proved to be accurate for thermometry up to 1143 K, limitations were observed at flame temperatures where temperatures were overestimated and relative CO(2)/N(2) concentrations were underestimated. Potential sources for these discrepancies are discussed.

High-energy, diode-pumped, nanosecond Yb:YAG MOPA system

Diode-pumped nanosecond multi-pass laser amplification to the joule level using an Yb:YAG slab crystal has been demonstrated. A maximum output pulse energy of 2.9 J at an optical-to-optical efficiency of 10% has been achieved. The seed pulses with a pulse duration of 6.4 ns were generated in a Q-switched Yb:YAG laser and amplified up to a pulse energy of 200mJ in a multi-pass booster amplifier. A maximum average output power of 15W at a repetition rate of 10 Hz has been measured. We also present a relay imaging semi-stable cavity for multi-pass amplification and a diode-pumping scheme employing horizontally stacked high-power laser diodes.

Image Relay for Object Naming during Awake Craniotomy

Image Relay for Object Naming during Awake Craniotomy

Anisotropy of nonlinear coupling of two counter-propagating waves in photorefractiveFe:KNbO3

The dependence of self-pumped photorefractive two-beam coupling in $\mathrm{Fe}:{\mathrm{KNbO}}_{3}$ with crystal angle has been measured at $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ for the $a$-$c$ and $b$-$c$ crystal planes using a combination of oil immersion and relay imaging. A significant deviation between existing theory and experiment is found for large crystal angles. We propose this is due to a highly anisotropic effective charge trap density in $\mathrm{Fe}:{\mathrm{KNbO}}_{3}$ of almost two orders of magnitude variation with crystal angle in the $a$-$c$ plane.

Rigorous coupled wave analysis applied to transmission efficiency of diffractive beam array relays for free-space optical interconnects

Parallel high-density free-space optical interconnects typically relay multiple channels in an array configuration; thus, they require good uniformity across their aperture for optimum performance. Rigorous coupled wave analysis is used to determine the throughput off-axis diffraction efficiency for Fresnel lenses within a diffractive imaging relay. The rigorous results are compared with scalar theory and show a significant nonuniformity not predicted by scalar theory. However, the polarization sensitivity is found to be negligible for the f-numbers considered (f/2.9 to f/10.2). These results are supported by experiment.

An aero-spacecraft for the far upper atmosphere supported by microwaves

Abstract We propose a new type of air/space craft, supported from below at altitudes ∼ 70 km by a ‘radiometric’ temperature-difference effect. From there it can perform communications relay, environmental monitoring, data telemetry and high-quality optical imaging. We show that an ultralight, mostly carbon-fiber “Lifter” is feasible using the radiometric force, an effect known since the 19th century, in combination with optimized properties of known carbon fiber materials. A powerful microwave beam illuminates the Lifter underside to provide the required temperature difference. Optimally, the full ambient atmospheric pressure can be delivered to one side of the Lifter area by heating it well above the ambient air at 200 K. Beam powers of ∼ MW can support masses of ∼ 100 kg . We verified the magnitude and pressure dependence of the effect on carbon fiber disks heated by microwaves in laboratory experiments. Numerical simulations suggest that carbon fiber craft of certain shapes can provide passive stability while riding a narrow microwave beam, and active stabilization is available through beam manipulation.

Generation of terawatt 10-fs blue pulses by compensation for pulse-front distortion in broadband frequency doubling

Pulse-front distortion as a result of broadband frequency doubling (BFD) has been compensated for over a 3-cm beam diameter by use of a telescope image relay system. As a result, distortion-free blue pulses with a peak power of 1.4 TW and a pulse width of 10 fs have been generated with a 10-Hz Ti:sapphire laser by use of this modified BFD configuration.

Aberration reduction by multiple relays of an incoherent image

Consider a generally aberrated one-dimensional (1D) optical pupil P illuminated by quasi-monochromatic light of mean wavelength lambda. In past work it was found that, if the pupil's intensity point-spread function (psf) is multiply convolved with itself, as in an imaging relay system, and then ideally (stigmatically) demagnified, the resulting psf s(x) approaches a fixed Cauchy form s(x) = deltax( pi2x2 + deltax2)(-1), which is independent of the aberrations of the pupil. Here deltax is the Nyquist sampling interval given by deltax = lambdaf/2 with f the f/number of the pupil. This Cauchy form for this intensity psf s(x) also manifestly lacks sidelobes. The overall questions that we examine are how far do these effects carry over to the case of a circular, two-dimensional (2D) pupil, and to what extent do practical imaging considerations compromise the theoretical results? It is found that, in the presence of spherical aberration of all orders, the resulting theoretical psf of a large number of self-convolutions approaches a "circular" Cauchy form, S(r) = 2deltar[pi2r2 + (4deltar/pi)2](-3/2), where deltar is the Nyquist sampling interval lambdaf/2 with f the f/number of the (now) circular pupil. Thus, for these aberrations the 1D effect does carry over to the 2D case: The output psf does not depend on the aberrations and completely lacks sidelobes. However, when all aberrations are generally present, the output psf s(r, theta) does depend on the aberrations, although its azimuthal average over theta still preserves the circular Cauchy form, as a superposition of Cauchy functions. Imaging requirements for achieving these ideal effects are briefly discussed as well as probability laws for photons that are implied by the above-mentioned PSF's s(x) and S(r). Real-time super resolution is not attained, since the stigmatic imaging demanded of the demagnification step requires the use of a larger-apertured lens. Rather, the approach achieves significant aberration suppression.

Comparison of adaptive optics and phase-conjugate mirrors for correction of aberrations in double-pass amplifiers.

Correction of birefringence-induced effects (depolarization and bipolar focusing) were achieved in double-pass amplifiers by use of a Faraday rotator between the laser rod and the retroreflecting optic. A necessary condition was ray retrace. Retrace was limited by imperfect conjugate-beam fidelity and by nonreciprocal refractive indices. We compared various retroreflectors: stimulated-Brillouin-scatter phase-conjugate mirrors (PCMs), PCMs with rod-to-PCM relay imaging (IPCM), IPCMs with astigmatism-correcting adaptive optics, and all-adaptive-optic imaging variable-radius mirrors. Results with flash-lamp-pumped, Nd:Cr:GSGG double-pass amplifiers showed the superiority of adaptive optics over nonlinear optic retroreflectors in terms of maximum average power, improved beam quality, and broader oscillator pulse duration/bandwidth operating range. Hybrid PCM-adaptive optics retroreflectors yielded intermediate power/beam-quality results.

Ultra-high-speed wide-angle catadioptric corrector system for medium-scale spherical mirror telescopes

The large amount of spherical aberration generated by a spherical mirror can be corrected by an image relay having a diameter much smaller than that of the pupil of the system. Described is a high-performance catadioptric corrector with a maximum input numerical aperture (NA) of 0.29 ( f /1.7), a fixed output NA of 0.67 ( f /0.75), and a passband of 405 to 1000 nm, that can be adapted to operate with spherical primary mirror systems having small to medium pupil diameters. Design examples are presented of systems with pupil diameters 0.5, 1, 2, and 4 m, with corresponding angular fields of 4, 2, 1, and 0.5 deg. The image in all examples is ~26 mm diameter, resolved to <4 μm root mean square (rms) over the whole field.

Correction of thermally induced birefringence in double-rod laser resonators – comparison of various methods

We compared various compensation methods for thermally induced birefringence in two-rod Nd:YAG laser resonators. The concept that lay behind these methods was 90° polarization rotation between two birefringent elements. In the first method, polarization rotation was done between laser rods, and in the second method, polarization rotation was done between passes through both rods. Both methods were tested with and without relay imaging between aberration planes. Thermal focusing was adaptively controlled by use of a variable radius of curvature mirror (VRM) or with an imaging VRM (IVRM). Best results were achieved with birefringence compensation between passes through relay-imaged rods and with the IVRM.

Simple integration technique to realize parallel optical interconnects: implementation of a pluggable two-dimensional optical data link.

An assembly technique is presented to realize pluggable or fully integrated optoelectronic systems based on image relays. A method to visually align and assemble optoelectronic chips or fiber bundles to half of a relay is explained. To validate this technique, two-dimensional arrays of vertical-cavity surface-emitting lasers and photodetectors and a fiber image guide have been integrated to gradient index lenses with simple optomechanical parts. Although the connection of these modules was realized with +/-0.5 mm lateral tolerances, parallel optical interconnects were successfully achieved at 10 MHz. The lateral misalignment between chips was on average 20 microm and at worst 60 microm.

Single spatial mode experiments on initial laser imprint on direct-driven planar targets

Flat plastic targets were directly irradiated and accelerated by partially coherent light from the GEKKO XII laser [Yamanaka et al., IEEE J. Quantum Electron. QE-17, 1639 (1981)] with the wavelength of 0.53 μm in order to investigate initial laser imprinting. The growth of the perturbation imprinted on the target by an initial foot pulse modulated with a single spatial frequency was observed by the face-on x-ray backlight technique. Imprint levels produced by the foot beam with a stationary intensity modulation of the illumination profile and with a dynamically changing modulation were successfully obtained by an image relay technique and the improved two-wavelength Young’s interference method. Simple analytic models are proposed and compared with the experimental results. Stationary imprinting with perturbation wavelength longer than the target thickness is found to be well described by a simple incompressible model. The dynamic dependence of the imprint on the time scale of the temporal illumination profile is found to be qualitatively explained by linear perturbation analysis.

Recombination laser in the exploding-type-pumping using a 30-nm thin membrane target

In order to increase the gain length product for the Ha line (13.4 nm) of hydrogen-like Nitrogen in the exploding type pumping of irradiating a 30 nm thick SiN membrane target with a few ps pulse, the pumping laser system is up-graded. For crucially important issue of suppressing a pre-pulse, the energy of the front-end of the pumping laser system was increased by one order. Beam pattern of the pumping laser pulse on the vacuum chamber window was improved with an image relay. In order to start experiments of a water-window laser, chirping pulse amplification in e-beam pumped KrF laser system is planned.